Conventionally, imaging technology has been centered around charge coupled device (CCD) image sensors. However, recently, CMOS imaging technology has become increasingly the technology of choice. There are a variety of reasons for this progression.
First, CCD imagers require specialized facilities, which are dedicated exclusively to the fabrication of CCDs. Second, CCD imagers consume a substantial amount of power, since they are essentially capacitive devices, which require external control signals and large clock swings to achieve acceptable charge transfer efficiencies. Third, CCD imagers require several support chips to operate the device, condition the image signal, perform post processing, and generate standard video output. This need for additional support circuitry makes CCD systems complex. Finally, CCD systems require numerous power supplies, clock drivers and voltage regulators, which not only further increase the complexity of the design, but also demand the consumption of additional significant amounts of power.
By contrast, CMOS imagers are characterized by a less complex design. The more simple architecture translates into a reduction in engineering and production costs and a concomitant and substantial reduction in power consumption. With today's submicron CMOS fabrication processes, CMOS imagers have also become highly integrated. For example, an entire CMOS-based imaging system, such as a digital camera, can be fabricated on a single semiconductor chip. Additionally, unlike CCD imagers, CMOS imagers are amenable to fabrication in standard CMOS fabrication facilities. This adaptability significantly reduces plant over-head costs. For these reasons, CMOS imagers are swiftly becoming the imagers of choice.
An image sensor is comprised of an array of picture elements or “pixels,” wherein each pixel comprises a photodiode or photosensor. A layout for an exemplary CMOS unit pixel 10 is shown in FIG. 1. Unit pixel 10 is comprised of a rectangular image sensing area 100, transfer transistor 102, floating node 104, reset transistor 106, drive transistor 108, select transistor 110, and output 112. Unit pixel 10 is powered by power supply VDD 114. Image sensing area 100 is made rectangular to maximize the “fill factor,” which is defined as the percentage of the unit pixel 10 area occupied by image sensing area 100. A typical fill factor for the arrangement of FIG. 1 is approximately 30%.
Referring now to FIG. 2, there is shown a plan view of a partial array of pixels 20, according to conventional CMOS image sensor devices. By positioning cylindrically shaped microlenses 203 over image sensing areas 200, the effective fill factor for the layout of FIG. 1 can be improved, as incident light 204 is focused more towards the center of rectangular image sensing areas 200 by microlenses 203. The percentage of each unit pixel 20 occupied by each image sensing area 200 does not, of course, change by employment of microlenses 203. Nevertheless, light capture is improved and the effective fill factor is increased. Use of cylindrically shaped microlenses 203 can increase the effective fill factor to approximately 75%.
Despite the improvement in effective fill factor using cylindrically shaped microlenses, there are negative performance factors, which are attributable to use of rectangular-shaped image sensing areas and cylindrically-shaped microlenses.
First, referring to FIG. 2, whereas utilization of cylindrically shaped microlenses 203 is effective at directing incident light 204 arriving at angles perpendicular to the major axes (major axes are in x-direction) of the lenses 203, the cylindrically shaped microlenses 203 are not very effective at focusing incident light 204 arriving at angles non-perpendicular to, i.e. oblique to, the major axes of the cylindrically shaped microlenses 203. This ineffectiveness further involves light that is scattered and/or reflected and which ultimately arrives at the lenses 203 at oblique angles.
The ineffectiveness of cylindrically shaped microlenses 203 to focus incident light towards the center of the image sensing areas 200 is problematic due to the fact that neighboring rectangular-shaped image sensing areas are in close proximity in the x-direction, where the horizontal spacing between neighboring pixels is shown to be approximately 0.8 μm. The close proximity results in random diffusion of photocharge generated outside the photodiode depletion regions. Photocharge that is generated outside the depletion region of a particular pixel is prone to capture by a neighboring pixel. When photocharge is unwanted captured by an adjacent pixel, electrical crosstalk occurs resulting in a reduction in image sharpness.
There is another type of crosstalk that can be attributed to the close proximity of neighboring rectangular-shaped image sensing areas 200. This second type of crosstalk occurs between pixels of different colors and is referred to in the art as “color crosstalk.” Color crosstalk leads to color distortion and is largely caused by the fact that silicon-based photodiodes have a wavelength-dependent photon absorption response.
In particular, color distortion can be significant for image arrays that use rectangular-shaped image sensing areas together with an RGB Bayer pattern color filter. In fact, a difference on the order of 10% in the response of GR, (green pixels adjacent to red pixels in the same row) and GB, (green pixels adjacent to blue pixels in the same row), under uniform illumination, is observed when rectangular-shaped image sensing areas are used. This difference in green pixel responsivity results in color distortion.
Finally, yet another problem that is observed when neighboring image-sensing areas are too closely positioned, is a decrease in spatial resolution. In imaging systems, resolution is quantified in terms of a modulation transfer function (MTF). The lower the MTF, the less capable an imaging device is at picking up the fine detail and contrast in the object that is being imaged.
In furtherance of the above discussion, a digital imaging system for photographic, video, or other imaging applications generally includes two major components, a solid-state imaging device and a taking lens. Solid-state imaging devices, such as charge-coupled devices or CMOS image sensors, are typically constructed as two-dimensional arrays of photo-sensing elements, i.e., pixels. In an example CMOS image sensor, each CMOS pixel usually contains a photodiode as the sensing element, and this sensing element is typically embedded in a circuit with several MOS transistors. The other important part of the imaging system is taking lens. A taking lens is typically constructed of a multi-element glass, plastics, or glass-plastic hybrid lens assembly and generally employed in front of the imaging device to focus the light onto the sensor array.
The performance of an imaging system strongly depends on the light collection efficiency of the sensor. For the aforementioned example CMOS image sensor, the light collection efficiency is determined by the ratio of the photodiode area to the total pixel area, called “fill factor.” A large fill factor is generally desired to increase the optical energy incident on the pixel that can be collected by the photodiode. However, the maximum achievable fill factor is limited by the CMOS fabrication design rules between photodiodes and MOS transistors. An example of this is illustrated in FIG. 3.
As illustrated in FIG. 3, an area of silicon 30 corresponding to a pixel area or site contains a photodiode 35. This pixel site or area is usually covered by a dielectric 40. Incident light 50 from an image to be captured passes through the dielectric layer 40 and impinges upon the area of silicon 30 containing the photodiode 35. As further illustrated, not all of the incident light 50 impinges upon the photodiode 35, thus the ratio of incident light 50 to that light which impinges upon the photodiode 35 is the light collection efficiency.
As the pixel size (300 & 400) is scaled down, as illustrated in FIG. 4, to reduce the die size or increase the sensor resolution with a fixed die size in order to achieve some financial benefits, the area of the photodiode 350 needs to be shrunk further in accordance to the layout design rules. As a result, the fill factor is reduced and the sensor performance is degraded.
In order to compensate for the degraded light collection efficiency for small fill-factor pixels, a microlens 60, as illustrated in FIG. 5, has conventionally been placed over a pixel (300 & 400) to refract light hitting on the surface of the pixel (400) into the photodiode 350. As shown in FIG. 5, the microlens 60 concentrates light 500 from the taking lens onto the photodiode 350, improving light collection efficiency and increasing the effective fill factor.
The effectiveness of the microlens 60 is influenced by the telecentricity of the taking lens. For slim cameras with thin lens assemblies, the taking lens 90 tends to be non-telecentric, as illustrated in FIG. 7. That means the light rays 85 will impinge on the sensors 70 with a large incident angle. On the other hand, using a telecentric lens 80, as illustrated in FIG. 6, the light rays 82 impinge on the sensors 70 with a small incident angle. The angle is typically measured using chief rays, as shown in FIGS. 6 and 7. In typical lens considered for ⅓″ SXGA sensor applications, the chief ray angle is 12°-23°.
The effect of a large chief ray angle, as illustrated in FIG. 7, on an array with microlenses is significant. The incident angle causes the focus spot to shift away from the photodiode site as a function of pixel distance from the optical center of the array. This phenomenon is demonstrated in FIGS. 8 and 9.
As illustrated in FIG. 8, incident light 550 from a telecentric lens impinges upon a microlens 60 that focuses the light through a dielectric layer 40 onto a pixel site 30 in which a photodiode 35 is located. Since the angle of the incident light 550, as illustrated in FIG. 8, is small, the light collection efficiency is high. On the other hand, in FIG. 9, when using a non-telecentric lens, the incident light 560 impinges upon a microlens 60 that focuses the light through a dielectric layer 40 onto a pixel site 30 in which a photodiode 35 is located. As illustrated in FIG. 9, since the angle of the incident light 560 is large relative to FIG. 8, the microlens 60 shifts the focused light to a corner of the pixel site 30 so that the light collection efficiency is significantly reduced.
Therefore, it is desirable to provide an imager in which a non-telecentric lens can be used without reducing the light collection efficiency of the pixel site.